This work is aimed at establishing a quantitative, physical understanding of membrane dynamics and thermodynamics using modern techniques of electron spin resonance (ESR) spectroscopy recently developed in the laboratory of Freed, and spin-labels closely resembling natural membrane components. Specifically: (1) The new and general method of using ESR-determined orientational order parameters to measure the activity coefficients of model membrane components will be applied to a variety of systems to investigate the thermodynamics underlying (a) the preference of cholesterol for certain lipids in lipid "blends", (b) the role of lipid asymmetry in biological membranes, and (c) lipid/polypeptide interactions in membranes. (2) Dynamic ESR imaging will be used to make highly accurate macroscopic measurements of lateral diffusion in biomembranes. The fundamental relationship between diffusion coefficients and orientational order parameters will be used to quantify the correlation between membrane structure and fluidity. (3) Macroscopic diffusion in membranes will be compared with microscopic relative diffusion measured using new 2D pulsed methods and recently developed spectral-spatial ESR imaging methods. These studies will be directed towards understanding the dynamics of molecular clustering in mixed model membranes. (4) The internal configurational dynamics of lipid chains will be studied in various systems using modern ESR techniques in combination with a new theoretical model in order to detail the effects of membrane components on lipid chain dynamics. (5) Lipid interactions with polypeptides will be studied using the enhanced resolution of new time-domain and frequency-dependent ESR methods, which will provide better discrimination between bulk lipids and boundary lipids, and a direct means of measuring exchange rates between the two regions. This research is directly relevant to studies of membrane morphology, and thus could particularly benefit a wide variety of disciplines where it is important to probe the physical response of membranes. Possible clinical applications include the study of transmembrane drug delivery mechanisms, detection of membrane changes during the immune response, and diagnosis of membrane disorders in diseases of the blood, lungs, liver, and kidney.